BOV Question 4

[howboycat]

I was reading more into my Corkey Bell book and noticed there was no mention of a blow off valve. The only means of venting boost from the pressure side of the system he mentions is a "vent valve". I assume this "vent valve" is meant to be the BOV.
About the "vent valve" he states. "A Rather sophisticated radiator cap can be used as a boost-control device. Generally, these types of controls will prove inaccurate and often noisey."
Now am I mistaken or is the primary need for a BOV to release boost pressure when the throttle is snapped shut.
And is it not a necesity because the wastegate alone cannot reduce the pressure fast enough during this particular driver action ?

 

[patprimmer]

Ummm

What's a BOV or vent valve or inlet pressure relief valve or whatever someone chooses to call it got to do with a waste gate.

Regards

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[howboycat]

I'm just trying to clear this up. Corkey Bells writing makes it sound as if you can throw the BOV away if you have a wastegate. Like their two seperate ways of doing the same job and not to be used in conjunction.

 

[Bribyk]

A wastegate can't dump intake air that has already been pressurized by the compressor wheel when the throttle snaps shut...that's what the BOV is for.

 

[SomptingGuy]

A wastegate can't dump intake air because it's not connected to the intake.

- Steve

 

[turbomotor]

cyclecyko,

A "blow-off valve" is an intake manifold vent valve that bypasses air from the intake manifold to either the air duct upstream of the turbocharger, or sometimes overboard.


While the action of a BOV is very much effected by the exhaust bypass valve
(wastegate), the purpose is not the same.

The wastegate is typically used for primary boost (or, preferably, manifold absolute pressure) control.

The BOV is typically intended to prevent compressor stage "surge" or backflow that occurs when the compressor wheel stalls aerodynamically and can no longer support the downstream static pressure.

Compressor surge can be violent enough to shorten the life of the turbo-compressor axial thrust bearing, hence the hot-rodders and aftermarket turbo engine modifiers have employed a BOV valve.

 

[SonyAD]

I think the need for a BOV is only an issue on throttled engines. Petrol spark ignition engines and anything else that requires stoichiometric air/fuel ratio achieved through use of a induction choke or throttle plate.

Unthrottled engines can probably easily accommodate the pressure already in the intake manifold before they've revved down significantly and as such only require a wastegate.

Why not have the engine management system leave the throttle plate fully or partially open and cut injection entirely instead of using a BOV?

 

[mattsooty]

Surely its obvious that a BOV is only necessary on a throttled engine - isnt it the very action of closing the throttle that causes the over pressure in the intake system???

The same effect cannot possibly be replicated with a change in injected fuel mass on a CI engine.

Leaving the throttle open & killing injection might work but I think the software functionality would be very poor from a driveability point of view.

MS

 

[SonyAD]

Because of weak engine braking?

 

[70btdc]

The radiator-cap idea was used as a primitive boost control device back in the day. Of course any intake pressure that is released this way is a waste of the energy spent compressing it. Also the turbo will not be regulated in terms of max speed and must be sized accordingly to run without a wastegate i.e. too large. In effect you can use the pop-off valve idea to control boost, and I believe Corky Bell does suggest exactly that.

Blow off valves can also be set up to achieve this function if the spring is set to equal max desired boost pressure. In fact some of the cheaper BOV's start to leak at higher boost levels, people thinking that they have max'ed the turbo instead when the boost curve goes flat.

 

[Silverbullet86]

There are different designs of BOVs. A "pull type" BOV will not leak due to the fact that having + pressure in the charge pipe keeps the BOV closed, while a traditional "push type" BOV will have the + pressure from the charge pipe pushing against the piston towards the direction is opens.

i dont know why so many times ppl try to relate a BOV to a waste gate. they hve nothing to do with each other. One is used to control the amount of exhaust gas going into the turbine (wastegate), and the other is used to prolong the life of the compressor by limiting the amount of compressor stall (BOV)

 

[Silverbullet86]

Also wanted to add, that many turbo chargers on the market today are build much much stronger than before and it may not even be necessary to use a BOV in attempt to gain throttle response. Some manufactures sell anti surge compressor housings.

 

[JonKBmw]

The BOV is not for throttle response at all. Its to vent pressure that would otherwise surge backwards through the compressor when the throttle is closed. "Strength" has nothing to do with it - anti-surge (or map-enhancement) ports do, but all turbocharger applications require either a blow off valve or a bypass valve, and if you fail to use one you WILL surge the compressor and this will eventually damage your turbocharger which could lead to catastrophic engine damage if the turbo lets go. Even with an anti-surge housing you still need a BOV - there are two types of surge: the kind that occurs when you shift and the throttle is shut and you hear the "ch-ch-ch" as your wheel spins backwards and forwards, and then the kind that happens when running a large compressor on an engine with low flow. The latter is what an anti-surge port is for.

 

[izzmus]

"All turbocharger applications require either a blow off valve or a bypass valve"

Except of course draw-through arrangements and unthrottled engines, which couldn't use a valve of that type if you tried.

And, please inform Ford, Subaru, Isuzu, and probably more manufacturers that the thousands of thousands of turbo engines they produced in the 80's were faulty because they didn't have those devices. At least, the Subaru turbo I had did not have one, the Isuzu turbo I had did not have one, and none of the Ford 2.3 turbo engines I have seen had one. I understand that Mazda's first turbo rotary also did not have one, but the second one *did* have one purely for NVH reasons.

 

[Tmoose]

I believe on positive displacement, non-centrifugal superchargers a BOV is meant to try to prevent this -

http://www.hotrodchat.com/albums/album52/blower_explosion.jpg

For decades those heavy retention straps have been required, since no BOV is effective at relieving this kind of hiccup

http://www.marshu.com/images-websit...s-crashes-accidents/funny-car-Del-Worsham.jpg

 

[Bribyk]

They use an aluminum burst plate in top-fuel cars (non-cent supercharger) because a "BOV" can't react quick enough when an intake valve fails to close. It's got little to do with normal blower operation.

 

[Silverbullet86]

... and then the kind that happens when running a large compressor on an engine with low flow. The latter is what an anti-surge port is for.

I'm not sure if i understand that statement. Define low flow. Isn't the point of boosting an N/A motor to make it flow more: ie it had low flow, and now flows higher.

 

[SonyAD]

I agree with izzmus. My turbodiesel gets along quite well with just a wastegate to prevent overboost.

 

[izzmus]

The anti surge ports/chambers/whathaveyou basically help extend the effective range of the turbo "left" on the map. Low flow in this case means flow lower than the (normal) minimal amount of air flow that the turbo can handle at a given pressure ratio.

 

[Deltona]

I think that if the full paragraph from CBs book was quoted it would have saved a lot of trouble:

"Vent Valve:

A Rather sophisticated radiator cap can be used as a boost-control device. Generally, these types of controls will prove inaccurate and often noisy. While far superior to any form of restrictor (referred to earlier in the chapter), these valves probably have their greatest value as safety controls in the event of wastegate failure. They can be commonly found on production turbo cars as overboost safety features. The vent valve has no business being a primary boost control device."

This is 1970's technology, the ECU has been controlling the boost on engines since the mid to late Eighties....

These statements:

"The BOV is typically intended to prevent compressor stage "surge" or backflow that occurs when the compressor wheel stalls aerodynamically and can no longer support the downstream static pressure."

"Compressor surge can be violent enough to shorten the life of the turbo-compressor axial thrust bearing, hence the hot-rodders and aftermarket turbo engine modifiers have employed a BOV valve. "

"Even with an anti-surge housing you still need a BOV - there are two types of surge: the kind that occurs when you shift and the throttle is shut and you hear the "ch-ch-ch" as your wheel spins backwards and forwards, and then the kind that happens when running a large compressor on an engine with low flow. The latter is what an anti-surge port is for. "

"And, please inform Ford, Subaru, Isuzu, and probably more manufacturers that the thousands of thousands of turbo engines they produced in the 80's were faulty because they didn't have those devices"

Are all untrue according to some schools of thought, that aside you will never ever see a compressor wheel spinning backwards and forwards! All manufacturers use BOVs or dump valves (DV) as we call them in the UK and always have done, the reason? Because the engine makes a weird embarrassing noise when they don't have one fitted, that's the only reason - it's noise control.

I have driven my own cars with no DV fitted, the turbo did not collapse, it did not slow down any further or speed up any quicker....

Do you think that a turbo WRC car uses a DV or BOV to release pressure on the gear changes? Nope.